We propose to continue to investigate the nature and possible mechanisms of alterations in energy utilization which occur with food restriction in the rat. We, and others have found that food restriction produces energy conservation in male rats, which serves to moderate body weight loss and to facilitate its restoration. These findings have important implications for humans who attempt to produce and maintain a reduction in body weight. The plan of these proposed studies is to study, in rats, the metabolic response to underfeeding in: a) females, who may respond to nutritional stress differently from males, and; 2) in 2 models of acquired obesity--spontaneous obesity which occurs with age in ad libitum feeding rats, and obesity produced by feeding rats a high-fat diet. Our goals are to: 1) determine whether males and females differ in their utilization of ingested energy during underfeeding; 2) determine whether acquired obesity alters the metabolic response to underfeeding; 3) determine whether the energy conservation produced by underfeeding can be lessened by changing meal patterning or diet composition, or by forcing the animals to exercise; 4) determine whether particular changes in circulating hormones or adipose tissue metabolic activities are related to the state of energy utilization. Alterations in food intake will be produced using automated food dispensors which allow delivery of small frequent meals. The resulting changes in energy utilization will be measured indirectly (by partitioning ingested energy into carcass storage, cost of storage, and energy required for maintenance) and directly (by 24-hr indirect calorimetry). Results of this work should help in understanding difficulties humans have in producing and maintaining a body weight reduction. Further, these results may identify, in rats, factors which can lessen the energy conservation produced by underfeeding and, thus, allow more successful maintenance of body weight at lower levels.